The determination of water content is important in many commercial products. For example, minute quantities of water in chemical process streams are detrimental for certain reactions. Further, the electrical properties of insulators are strongly dependent on water traces, and the water content of fluids such as gasoline has to be kept below a certain level. From these few examples, it is apparent that water determinations are among the most frequently performed analyses in many laboratories.
The currently most widely practiced water determination method is the "Karl Fischer" method, named after its originator who described the basis of this method in "Zeitschrift Fuer Angewandte Chemie", volume 48, p. 394-396 (1935). In this method, a sample containing an unknown amount of water is added to a Karl Fischer reagent, hereinafter denoted K.F. reagent. This reagent is usually a solution of iodine and a reducing ion (sulfur dioxide) in a buffer (such as pyridine or amines), and methanol or other solvents.
Due to the presence of iodine in the K.F. reagent its color is brown. When an unknown sample of water is added to the K.F. reagent, the iodine is consumed so that the dark brown color of the iodine in the "fresh" reagent disappears, changing to a light yellow color of the "spent" reagent. In a typical water determination by the Karl Fischer method, a water-containing sample is injected into a prepacked volume of K.F. reagent and the change in optical absorbance of the K.F. reagent is measured. The change in optical absorbance can be transformed by an electrical circuit into a direct readout of water content of the sample. This technique gives immediate quantitative results irrespective of the fading of strength of the K.F. reagent during storage.
An example of a reaction vessel for holding a K.F. reagent used in colorimetric analysis is that shown in U.S. Pat. No. 4,005,983. The vial of this reference includes a generally cylindrical shaped tube having an opening at one end which is covered by a liner held by a screw cap. An opening is provided in the screw cap for insertion of a syringe needle through the elastic liner. This allows entry of a sample into the vial while not exposing the K.F. reagent within the vial to the atmosphere. An electrical circuit is provided which compensates for the dilution effect that occurs when a sample volume is added to the prepacked K.F. reagent in the vial.
While the vial described in the aforementioned patent has many advantages, it does not solve all of the problems encountered in this technology. For example, the K.F. reagents prepacked in these vials have to remain in the vials for long periods of time, extending for many months and even years, prior to use. This places difficult burdens on both the inherent stability of the K.F. reagents and the design of the vial itself. Since the intrusion of even micrograms of moisture from the atmosphere will render useless the K.F. reagent contained therein, the vial must provide perfect sealing for very long periods of time. It has been found that the vials of U.S. Pat. No. 4,005,983, while providing adequate sealing for a few months, do not provide prolonged sealing sufficient to prevent all moisture from contacting the enclosed K.F. reagents.
Another requirement of the vial is that there must be easy access to the interior of the vial without, as noted above, exposure of its contents to the atmosphere. While the vials of U.S. Pat. No. 4,005,983 allow easy introduction of the sample, they do not provide the proper sealing, primarily due to leakage through and around the plastic septum (liner) used to seal the open end.
In water determination by colorimetric K.F. analysis, many variations can arise which will impact the accuracy of the result. For example, when a sample is added a dilution effect occurs. It is important that the dilution effect result in a predictable and uniform absorbance change. In addition to this dilution effect, the optical absorbance technique used for colorimetric K.F. analysis yields results which are dependent on the nature of the vials or containers that are used, the unknown properties of the added sample, and the inaccuracies produced by the use of glass or other types of transparent containers. For example, the walls of the containers in which the K.F. reagent-sample reaction occur are not generally optically perfect. This means that the second optical reading, after the addition of the unknown sample, may be changed by a difference in properties of the glass walls of the containers. Also, the optical properties of the added unknown sample, such as its optical density, refractive index, etc. are unknown at the time the sample is added to the reagent-containing vessel. Again, this will cause a change in the second optical absorbance reading which could render it inaccurate. Still further, another factor which impairs the accuracy of this technique is that containers are generally cylindrical vessels. Optical readings through these vessels are more complex than through perfectly rectangular optical containers. In order to address these variations which could lead to inaccurate results, a technique was proposed for colorimetric determination of water content using Karl Fischer reagents where the method involved using two different wavelengths for analysis. This approach is described in U.S. Pat. No. 4,786,602, wherein a small amount of a dye solution is included in the prepacked volume of K.F. reagent in the container. This dye was chosen to be one having its maximum absorption at a wavelength which is different than the wavelengths that are optically absorbed by iodine. Optical absorption measurements are made on the solution including both the K.F. reagent and the dye at two different wavelengths. These two different wavelengths are also used after the unknown sample is added to the K.F. reagent. Use of the second wavelength allows a correction for the reading determined by the first wavelength in order to compensate for the various factors causing inaccuracy in colorimetric K.F. water determination.
While the approaches described hereinabove have led to improved accuracies in colorimetric Karl Fischer water determination, additional improvements need to be made. For example, it is desirable to provide an improved vial in which a known amount of Karl Fischer reagent can be stored for an indefinite amount of time, where the vial is one which is easy to use for insertion of the unknown sample and which provides a perfect seal for an extended period of time prior to its use. Further, many existing vial designs do not allow entry of a predetermined precise amount of sample, where the sample volume is extremely small. This can impose an additional burden on the vial design since a substantially perfect seal also must be provided before, during and after introduction of the unknown sample into the vial. The prior vial designs (with the exception of that described in U.S. Pat. No. 4,005,983) utilize ratios of sample: reagent of at least 5:1. In contrast with that, the sample: reagent ratio in the vials of the present invention are less than about 1:4. Vacuum and capillary action to introduce the sample are not suitable for very small sample amounts.
Accordingly, it is an object of this invention to provide an improved vial that can be used for the colorimetric determination of water content by the Karl Fischer technique.
It is another object of this invention to provide an improved vial having a predetermined amount of Karl Fischer reagent therein, where the vial provides excellent sealing against an ambient atmosphere of any type, before, during and after introduction of the unknown sample into the vial.
While it is important to provide a vial providing excellent sealing before, during and after entry of the sample therein, it is also important to provide a vial into which the sample can be introduced in precisely determined amounts. It is sometimes preferable that the burden of limiting the amount of sample introduced into the vial not be placed on the vial itself.
Thus, it is another object of the present invention to provide an improved design of a vial useful for a colorimetric determination of water content by the Karl Fischer method, in which the sample is readily introduced into the vial in predetermined amounts not directly determined by the vial design.
It is another object of this invention to provide an improved vial useful in colorimetric K.F. water determination where a predetermined amount of unknown sample is easily introduced into the vial.
In addition to the requirements on the vial, described above, it is also advantageous to provide improved K.F. reagents which are particularly adapted to the Karl Fischer determination of water content by colorimetric analysis. Prior to the present invention, there has been little effort to define Karl Fischer reagents which would be particularly suitable for colorimetric analysis. It is recognized, however, that many different reagents have been used for Karl Fischer determinations, and in particular for coulometric analysis techniques. However, the art has not focussed on the development of reagents which will yield linear changes of absorbance when adding water, or reagents which will provide predictable and uniform absorbance changes when the solution is diluted by the addition of the unknown sample. In the present invention, it has been found that the addition of particular additives in selected amounts will provide reagents that are advantageous for colorimetric analysis via the Karl Fischer technique, and are particularly suitable for use in the vials of this invention. These reagents provide linear changes with the water content of the unknown sample and also very controlled and uniform changes in optical absorbance due to the dilution effect that occurs when a volume of sample is added to the reagent.
Thus, it is another object of my invention to provide improved reagents for the K.F. determination of water by colorimetric analysis.
It is another object of my invention to provide improved reagents useful for the colorimetric analysis of water content using the Karl Fischer technique, where these reagents are used with the improved vials described hereinabove.
It is another object of my invention to provide a sealed, improved vial having contained therein a prepacked volume of the improved K.F. reagents of this invention.
It is another object of my invention to provide a process for K.F. determination of water content in an unknown sample using colorimetric analysis involving the use of improved reagents in vials described herein.
In Karl Fischer analysis for water determination titrimetric methods are commonly used wherein the K.F. reagent is a monocomponent reagent or a bicomponent reagent. In the first instance, the sample is titrated with a solution which contains a reducing ion such as SO.sub.2, a buffer such as pyridine or an amine, iodine, and an alcohol such as anhydrous methanol. Due to stability problems with extended storage, a bicomponent reagent is typically used wherein the vessel solution contains sulfur dioxide and pyridine in methanol while the titrant contains iodine in methanol. Examples of reagents suitable for titrimetric determinations of water are described in U.S. Pat. No. 4,378,972 wherein several amines can be used.
While there are many reagents known in the art for use in titrimetric determinations of water using the Karl Fischer method, there has been relatively little effort to develop reagents that are particularly suitable for colorimetric determinations of water content using the Karl Fischer method. Reference is made to U.S. Pat. Nos. 3,723,062 and 4,786,602 for a description of K.F. reagents previously used for colorimetric determinations of water content.
Colorimetric water determination is quite different from titrimetric water determination and it is apparent to those of skill in this art that the teachings from one technique cannot be automatically applied to the other technique. For example, many of the reagents used for titrimetric water determinations cannot be used for colorimetric water determinations. For example, as will be described later, the iodine content of a colorimetric reagent is preferrably an order of magnitude less than it is in a titrimetric solution.
Thus, it is a further object of this invention to provide improved and new reagents for colorimetric determination of water content using the Karl Fischer analysis.
It is another object of this invention to provide improved reagents for colorimetric determination of water content by the Karl Fischer method, where the reagents have improved optical properties.
It is another object of the present invention to provide a combination including a sealed vial of a particular design having therein a Karl Fischer reagent that can be used for colorimetric analysis of water content.
It is a further object of this invention to provide a process for colorimetric determination of water content using a sealed vial of a particular design having therein an improved colorimetric K.F. reagent.